Nucleation in sheared granular matter (1705.02984v4)

Abstract: We present an experiment on crystallization of packings of macroscopic granular spheres. This system is often considered to be a model for thermally driven atomic or colloidal systems. Cyclically shearing a packing of frictional spheres, we observe a first order phase transition from a disordered to an ordered state. The ordered state consists of crystallites of mixed FCC and HCP symmetry that coexist with the amorphous bulk. The transition, initiated by homogeneous nucleation, overcomes a barrier at 64.5% volume fraction. Nucleation consists predominantly of the dissolving of small nuclei and the growth of nuclei that have reached a critical size of about ten spheres.

• The paper demonstrates a first-order phase transition from disordered to ordered granular packing beyond a 64.5% volume fraction.
• It employs cyclic shear on nearly 50,000 glass spheres to trigger homogeneous nucleation with a critical cluster size of about ten spheres.
• Statistical analysis reveals distinct FCC and HCP crystalline regions coexisting with amorphous structures, challenging traditional nucleation theories.

Nucleation in Sheared Granular Matter

The paper "Nucleation in Sheared Granular Matter" presents an experimental paper on the crystallization process of macroscopic granular spheres under cyclic shear, a system that serves as an analogue to thermally driven atomic or colloidal systems. Through a methodical examination, the authors document a first-order phase transition from a disordered to an ordered state in the granular packing, marked by the formation of crystalline clusters exhibiting mixed face-centered cubic (FCC) and hexagonal close-packed (HCP) symmetries. This transition notably occurs when the volume fraction surpasses the threshold of 64.5%.

Experimental Setup and Observations

The experimental apparatus consists of a cubical shear cell containing approximately 49,400 precision glass spheres drenched in a refractive index-matched liquid. Through the application of cyclic shear, the authors observed a phase transition initiating from homogeneous nucleation, a process distinguished by the critical nucleation size of approximately ten spheres. This is significant because it sheds light on the fundamental mechanisms driving the nucleation and growth processes in granular materials.

Throughout the experimental process, three primary phases of behavior were identified:

1. Initial Compaction: The granular packing begins in a disorganized state, compacting logarithmically over approximately 20,000 shear cycles.
2. Plateau Phase: The system reaches a volume fraction plateau at 0.645, corresponding to the so-called random close-packed state (RCP).
3. Crystallization Phase: Nucleation begins to occur beyond this plateau, wherein nuclei grow predominantly if they exceed the critical size, facilitated by the ongoing shear cycles.

Numerical Results and Analysis

A detailed statistical analysis of local packing densities reveals the coexistence of crystalline and amorphous regions, with local volume fractions bifurcating around a peak emerging at the density of crystalline structures (0.74). This dual peak distribution demonstrates the clear structural differentiation between the nucleated crystalline regions and the surrounding amorphous material.

The research quantifies the growth probability based on nucleus size, with experimental data indicating a positive nucleation progression for clusters larger than the critical size of ten spheres. This evidence supports the concept of a nucleation barrier within the granular system, which has implications for understanding phase transitions in other athermal systems under shear.

Implications and Future Directions

The insights from this paper advise a reevaluation of nucleation phenomena in granular matter, particularly under shear-induced conditions. The observations challenge some of the assumptions derived from thermal and Brownian systems, suggesting distinct properties in nucleation dynamics for granular materials. This research could significantly impact industries relying on granular compaction processes by enhancing the understanding of nucleation under mechanical agitation.

Future research could explore variations in parameters such as particle size distribution and packing geometry, as well as the influence of different shear intensities and frequencies. Extending this paper to numerical simulations with larger datasets could yield further insights to elucidate the critical mechanisms at play during nucleation in sheared granular systems.

Overall, the thorough experimental approach provides a robust framework for examining nucleation phenomena in granular materials, with critical implications for theoretical modeling and practical applications in material science.